This invention related to CO2 capture and more particularly to a sorbent capable of capturing CO2 at warm temperatures.
There is a growing consensus that global warming is caused by anthropogenic CO2 emission1. Among all sources, a coal-fired power plant represents the largest point source for CO2 emission2. But at the same time, coal is the most inexpensive and most abundant fossil fuel3. Thus, in the near future, although renewable energy and nuclear energy will play increasing roles, coal will remain a major component of the energy portfolio, especially in developing counties3. Facing this dilemma, technologies are needed to utilize coal with higher energy efficiency and lower CO2 emission. Currently, integrated gasification combined cycle with carbon capture and sequestration (IGCC with CCS) is one of the most promising candidates for achieving this goal4. IGCC has two important features compared with a traditional pulverized coal (PC) plant5. First, because of the combined cycle, IGCC can achieve higher energy efficiency. Second, the gas stream has higher pressure which facilitates the capture of sulfur, mercury, nitride oxide, particulate and CO2, making emission control less expensive.
Currently, the state-of-the-art CO2 capture process for IGCC is scrubbing the gas stream with physical or chemical solvents, such as MEA, Rectisol or Selexol. These solvent-based absorption processes need to operate at fairly low temperatures. Thus, the gas stream coming from a water gas shift reactor must be significantly cooled down, which leads to high energy loss and high capital costs both for the compressors used for cooling and for heat recuperation. A study carried out by DOE/NETL5,6, which assumed Selexol for carbon capture, indicated a reduction in net power output and a corresponding reduction in HHV thermal efficiency of 6%-9% as compared to a base case of IGCC with no carbon capture. It would be very desirable to reduce the parasitic energy load and operating costs associated with the traditional low-temperature CO2 capture process. Prior studies have assessed high temperature CO2 separation processes7,8, such as H2-permeable membranes9,10, CO2-premeable membrane11, and pressure swing adsorption (PSA)12, and identified opportunities to significantly improve the efficiency. A more recent study13 in our group extended previous analyses to compare a variety of novel separation methods for IGCC with CCS, namely sorbents and membranes, on a unified basis. Our computational approach identified pressure swing adsorption operated at warm gas temperatures (200-300° C.) could be potentially more efficient compared with other approaches. However, no sorbent was yet reported to be applicable in this temperature range.
Widely used in the oil and chemical industry, pressure swing adsorption20 (PSA) of gases onto solid sorbents provides some key advantages, such as low energy requirements, low costs, and ease of applicability. In spite of these advantages, there have only been a few studies related to regenerable sorbents of CO2 at 200-300° C. in the literature. To be applicable to a PSA process in the desired temperature range, the sorbent needs to maintain a regenerable sorbent capacity, fast kinetics, and low heat of adsorption. The commercially available sorbents such as activated carbon, zeolites, and alumina lose their adsorption properties at temperatures higher than 150° C., Super activated carbon21 can maintain sufficient capacity at temperatures as high as 220° C., but the CO2/N2 selectivity is low. Basic zeolites22,23 obtained by doping with electropositive ions were also tested for this purpose, hut showed poor performance in the presence of other polar gases such as SO2 and steam.
Some inorganic materials have also been proposed in the literature for carbon capture at an elevated temperature, such as calcium oxide24,25, lithium zirconate26, lithium silicate27,28, sodium-based sorbent29, hydrotalcite-like compounds (HTls)30,31 and double salt sorbent32. Calcium oxide demonstrated high capacity even, at 700° C., but suffered from poor regenerability, slow kinetics and an extremely large heat requirement. Lithium-based materials can capture CO2 in the 450-550° C. range, but suffer from slow sorption kinetics. Sodium-based materials showed good adsorption in the 200-400° C. range, but the materials can only be regenerated at 700° C., and so are not suitable for pressure swing adsorption. Double salt sorbent shows extremely high capacity at high temperature, but it is very hard to produce reproducible samples33. Recently, DOE/NETL reported a magnesium hydroxide based sorbent19, which has a large capacity in the 200-300° C. range, and can be regenerated at 375° C. But the re-generation via thermal treatment is still not desirable for our purpose. In addition, the breakthrough curve of this sorbent indicates slow sorption kinetics which could arise from the very low surface area of the sorbent. Among such inorganic materials, HTls stand out as the most promising candidate for warm CO2 capture, and they have been widely studied for sorption-enhanced water gas shifter reaction34,35. Hydrotalcite-like compounds consist of positively-charged brucite (MgOH)-like layers balanced by hydrated anions. The most common type is Mg—Al—CO3, in which stacked layers of magnesium hydroxide [Mg(OH)2] in which some of the divalent cations (Mg2+) are substituted by trivalent cations (Al3+) at the centers of octahedral sites of the hydroxide sheets. HTls are often promoted with K2CO3 to improve their performance in the presence of high pressure steam. These two classes of materials after calcination have been widely studied for CO2 capture at high temperatures (350-500° C.).
This patent application has as an object to develop a new sorbent for CO2 with good regenerability, fast kinetics and low heat of adsorption that can be applied in a PSA process in a warm temperature range. The material prepared through incipient wetness impregnation has a large surface area and pore size which facilitates the rapid adsorption of CO2. Through the detailed study of sorbent capacity, multi-cycle regenerability, sorption rate and comparisons with HTls, we demonstrate here that the sorbent disclosed herein is a candidate for warm CO2 capture by the PSA process.